Dual mikuni's 28's running rich

[Bob Wood]

I finished my XL last week and been running it up working out bugs and setting carbs. I swapped out the stock 200 jets for 170's per Scott Casler and other forum recommendations. My motor. Is a 37 hp Mosler. I get 3100 static (54x22 TN prop)and seems to run very well even at very low idle.Timing and valves set right on..my question is black sooty plugs really sooty black on left and a little less on the right. I left the jet clips in the middle position as I received them did not change them when I put in the 170,s. I started with the mixture screws out 1.5 turns now at 1 turn out . I am thinking I need to move the jet clips up to lean this thing out. Do I go to the first or second notch and keep the mixture screws out 1.5 turns , or is there a system to do this. From my motorcycle work I am thinking I need to end up about 1.5 out on the mixture and leaner on the needle clips. After I get good looking plugs I will fine tune with the EGT .
Any direction here will be appreciated

Bob

[Steve]

I finished my XL last week and been running it up working out bugs and setting carbs. I swapped out the stock 200 jets for 170's per Scott Casler and other forum recommendations. My motor. Is a 37 hp Mosler. I get 3100 static (54x22 TN prop)and seems to run very well even at very low idle.Timing and valves set right on..my question is black sooty plugs really sooty black on left and a little less on the right. I left the jet clips in the middle position as I received them did not change them when I put in the 170,s. I started with the mixture screws out 1.5 turns now at 1 turn out . I am thinking I need to move the jet clips up to lean this thing out. Do I go to the first or second notch and keep the mixture screws out 1.5 turns , or is there a system to do this. From my motorcycle work I am thinking I need to end up about 1.5 out on the mixture and leaner on the needle clips. After I get good looking plugs I will fine tune with the EGT .
Any direction here will be appreciated

Bob

Bob,
I think Joe Spencer is running a 160 & a 150 main jet on his 45 which requires more juice than your 37... Leave the Circlip in the center slot for now... Your rings may not be fully seated yet and that leaves the compression down some and a little oil getting into the combustion... Put in 160 jets & run it some more -

Joe is traveling today and will weigh in later for sure...
Steve

[rockiedog2]

yeh that's right a 150 and a 160. I would expect the 37 to need several sizes smaller
do you have dual egt? the power comes on above 1300...1350-1400 is where mine sings. since we don't have much power we need all we can get out of it and EGT was the only way I could reliably get max tune out of it.

[rockiedog2]

disregard the egt question I see you have those.  joe

[Steve]

This graphic is central to understanding a properly sized Mikuni VM carb

A new Edition of the Mikuni VM Tuning Manual is attached:

[Sam Buchanan]

Running mogas seems to result in sooty looking plugs regardless of jetting (unless extremely lean?...but it probably won't run well if it gets very lean...). Also expect to see sooty exhaust stacks. EGT is probably the only accurate way to sort out jetting.

Adjust mixture by ear to get smooth idle and smooth transition from idle. Your engine won't spend much time running on the needles, it will be near full throttle most of the time. If the needles are off, you won't hurt the engine.....it'll get your attention by misfiring......  

Unlike 2-strokes, these engines are bullet-proof even if jetted too lean as long as the timing is in spec.

[rockiedog2]

This graphic is central to understanding a properly sized Mikuni VM carb

A new Edition of the Mikuni VM Tuning Manual is attached:

I haven't studied the size chart in detail but do wonder from personal experience with the 45/vm28s if the 28s might be too large for the 37 horse for optimum power and efficiency. We should find out with this 37 and it's EGTs

[Steve]

I have felt for some time that the Mikuni VM26-626 (a popular Japanese made alum body Mikuni) is the better choice for the smaller motors 32-37 HP...

Let's see how Bob does with his 37 hp motor...

[Bob Wood]

So the process is to go down from my current 170 jets to 140. 150  Or 160. Until I can get the 1300
To 1400 on the EGT. I must be so rich and  cool now with the 170 jets because my EGT does not even hit the 900 min on my gauge. If I start with a jet too small say 140 will the engine just get doggy at high rpm due to lack of fuel?  
Thanks for the help on this!!
Bob

[Steve]

I'd put a 140 in the richer cyl and a 150 in the other - run it at different /varying power settings  2900 rpm - WOT for 1/2 an hour...

[Bob Wood]

Still at this jetting task to get to 1300-1400egt. With my mikuni 28 and my 37 hp I have a 140 jet on one side and a 150 on other. My needle clip Is in the middle I only show 500 on EGT and 1 plug very black, other showing some white porcelain .Running 92 no ethanol mo gas.  I have been running for about 20 min at 2000 -3100 rpm and see cht less than 250 and EGT 450-500.  Am I not running long enough?  My next step is to go one notch leaner on the needle clip to the second notch from top.   This is my first time at this is there a better procedure. My engine is broken in has about 40 hours on it. Last flown with single carb until my dual conversion.
Help, scratching my head here!,,

Bob

[rockiedog2]

i wouldn't change the needle yet.
where are your probes located? i run mine about 2-3" from the exhaust flange
it sounds like your egt setup may be suspect. are you sure you got the right probes for your gauge? if the thing is running atall it oughta be higher than 500 when the power is up. mine runs about 800 or so at idle. 
have you tried a smaller jet for the cyl with the black plug? i'd keep going down on that til i got them running more toward gray...just jet it with the plug color til you get it in the ballpark then fine tune it with the egt

[Bob Wood]

My EGT probe is 3 1/4" from flange....Maybe move it up an inch" to 2"...... I am running the Belite LED combo gauge for EGT and CHT on one side only. I was going to swap the probes over to get set up or put in 2 probes ans swap out at the gauge for set up. Belite recommends the falcon probes which I am running.

Since I am already down to 140 and 150 I figured I should move the needle clip. try 135 jets first maybe??

If I move the needle clip 1 notch leaner and it is too lean will it miss at full throttle?
Bob

[Bob Wood]

One other thing I forgot to mention, the Belite gauge has a minimum EGT temp of 900 and a min CGT of 250....As I said i am not hitting minimums on either. After 20 min or so of running I shut down and my CGT is like 230. with laser thermometer

[rockiedog2]

i never moved the clip on mine still in the middle
yeah i would just go smaller jets til got the plugs looking about right then adjust for the egt. i wouldn't be surprised to see the 37 end up with several size smaller jets than 150 i got a 150 on one side of my 45. with the low egts you got; assuming they're correct, i'd try 125s for hopefully an obvious difference in plug color/temps and adjust from there. since youre running one sooty plug i don't think youre on the lean side of peak and i wouldn't think 125s are gonna put you lean of peak either. but watch for that.  running premium youre not likely gonna make it detonate even if too lean. the comp is only 8:1 so you got all kinda margin. only reason Casler says premuim fuel is cause it's air cooled and runs at relatively high power routinely(as compared to say in a bug)
trial and error